I just wandered by with a big tub of popcorn and I think I’ll stay for a while.
SamuelA are you able to produce an fact-based argument to back up your assertions, or is your argument “what college did you go to?”. Responses to that are appropriately, I’d still like a fact-based argument explained coherently, followed by “what college did you go to”, and finally, my belief that you are not to going to the like the answer.
Let’s start with, please explain your position with citations.
In fairness, this isn’t a totally trivial problem and in order to short circuit a bit of too and fro, perhaps we can break it down.
The universe has a set of conservation laws. If we take our asteroid and the nuke sitting near it, and draw a box around the pair - say a box a few kilometres on a side (although it doesn’t really matter) we can say that within that box a set of properties must be conserved. Critically we are interesting in momentum and energy, but there is charge, spin, and so on.) Kinematics is governed by momentum and energy.
So, the next thing we do is convert some of the colour force binding energy in some of the nuke into a mix of kinetic energy - mostly in the form of very fast moving neutrons, a lot of highly energetic photons in the form of gamma rays, and a small amount of very fast moving atoms in the form of a plasma. Because momentum is conserved wrt to the nuke, the matter will all tend to expand away from the nuke in a spherical fireball.
So, there is a bunch of stuff, photons, neutrons and plasma that hits the asteroid.
Now within the confines of our box, we have two conservations laws we want to observe: energy and momentum.
Energy is messy, we can covert energy between all sorts of forms, but for kinematics we are only interested in kinetic energy. Momentum is much nicer. You can’t convert it into anything else, and the conservation laws provide an easy to work with set of constraints on the final outcome.
So, within our box, the sum of all momentum of all individual masses always just adds to the same value. Before we detonated our nuke we can define it to be zero. After, so long as our box is big enough to still contain everything that started inside the sum must still be zero. Momentum is a vector.
So, if the only thing that happens is that the momentum of the residue from the nuke that hits the asteroid starts the asteroid moving, the motion of the nuke is fully constrained by nothing more than this added momentum. We ignore the energy conservation question, and treat the interaction of the nuke and asteroid as an elastic collision. Total momentum of the system is conserved, as half the momentum of the nuke went out in the other direction into space, and so long as we take our snapshot quickly enough, the momentum inside the box remains zero. All good.
But we have ignored the energy, and essentially assumed it somehow was fully adsorbed by the asteroid, or vanished into space. However we have a huge range of options that might occur. The energy may be expected to vaporise a slab of the asteroid, and then as that expands, it pushes both on the asteroid and outwards into space. Momentum is conserved, the magnitude of the momentum transferred to the asteroid is the same as the magnitude of the momentum of the gas expanding out into space from the area of vaporisation. But note that we now have provided the asteroid with additional velocity, greater than the momentum that impinged from the nuke. We did this by moving mass on the asteroid with the energy we have from the nuke, but conserving momentum by having the mass move in different directions, some in some out.
Next we might notice that the vaporisation produces a massive shock wave that travels into the asteroid, this shock wave fractures the rock into small particles and the pressure of the expanding gas above will start to force the rock particles away from the centre of the blast impingement and they will naturally get forced at considerable speed is a curved path that eventually has them blast off the surface of the asteroid back in the the direction the initial blast came from, leaving a crater behind. Momentum remains conserved, the asteroid moves even faster away from the blast, with a velocity given by the balance of the mass of the ejecta and its velocity (the ejecta momentum) equalling he change in asteroid’s momentum. The interesting trick is that if we wish to maximise the velocity change of the asteroid, we want to maximise the momentum of the eject, all the time making best use of the energy - so minimising energy used.
energy = mv[sup]2[/sup]
momentum = mv
The upshot is that for a given energy budget we want to move the maximum mass with minimum velocity. But we can’t have the velocity of the eject so low that the ejecta remains gravitationally bound to the asteroid, so we must minimally exceed the escape velocity of the asteroid.
The point so far? There is no one momentum change of the asteroid. It can vary dramatically depending upon how well your nuke excavates and ejects mass from the surface.
The excavation process is thus critical. You need to maximise the transfer of kinetic energy from the nuke impingement to the rock directly below the point of impingement with as little loss as possible. First up you need to fracture the rock into small particles that will flow in front of the blast. This means you want solid homogeneous rock that transmits the shock wave with little loss and fractures as the wave passes. And you want more solid unfractured rock underneath the area of fracturing rock to support it as it flows and is ejected. The worst case is going to be and asteroid made up of some sort of aggregate of muck, one that is very lossy and simply adsorbs the energy of the blast and heats up, possibly just melting a big slab of the interior of the asteroid. Energy remains conserved, but now uselessly in the form of heat. Momentum remains conserved, as there was no ejecta, and so the asteroid doesn’t change velocity.
The question of maximising the momentum change of the asteroid is thus clearly one of maximising the efficiency of the ejection process. We need a process that ejects the maximum mass possible. Anything that reduces the mass ejected (even if the ejected mass contains a large amount of energy and is thus travelling fast) reduces the total velocity change of the asteroid. We can see that a critical part of this process is the manner in which the cratering process proceeds, and how this is governed by the properties of the shock wave in the asteroid. This can vary from high efficiency in a homogeneous solid asteroid, to very poor in an asteroid made up of an aggregate of junk.
A large asteroid may well be a gravitationally bound mess of junk, whilst smaller asteroids are probably more likely to be single entities, as they lack enough gravity to bind together a mess of junk. YMMV.
We don’t have to worry about large asteroids, since none of them come near the Earth. Some medium-sized asteroids are rubble piles, others are not; and asteroids with diameters of around 100 metres are too small and rotate too fast to be rubble-piles. So the nuclear option would work in quite a large number of cases. Once again, SamuelA is partially correct, despite the skepticism on this board.
How do we maximise the momentum transfer in such a process? Well, Freeman Dyson did some work on this concept, in the context of designing a space drive using atomic bombs. The nuclear devices in his concept include a significant quantity of propellant, and designed so that the force of the explosion was focused in a particular direction. Here’s a diagram from wikipedia.
These propulsive bombs are a very old concept, and may not be feasible in thi configuration, but the concept could be probably be refined in many ways.
The entire thread is however about large asteroids. Anything up to a 10km KT asteroid. The basic quote from Stranger on a Train that** SamualA** complains about is (bolding mine):
SamualA quotes this and goes on to say:
finishing with
SamualA does not seem to have ever looked as the questions of energy, and based his criticisms entirely on momentum conservation, with such comments as:
My point is simple. There is no, and has never been, an error involving conservation of momentum in any of what Stranger on a Train wrote. The discussion has been entirely about problems of maximising the impulse, and mechanisms by which energy goes into useless forms before being able to propel ejecta from the asteroid so that the momentum of that ejecta can add to the velocity change of the asteroid. This intermediate step has perhaps been missed in the condensed commentary that Stranger on a Train supplied, but it is somewhat unreasonable to jump from there to public accusations of professional incompetence without first putting in a few minutes background research to try to understand the topic.
OTOH, I have sympathy with an initial reading of the quote, as unless you start to look at the questions of underlying efficiency of using the energy from the nuke, it does read as if there is an error. But, like I say, it is always worth digging a bit further before accusing people of incompetence.
With the nuke as linked, that looks to be much the same device as the **Casaba Howitser **that Darren Garrison linked to earlier. This design is not intended to maximise momentum transfer. It is intended to create a highly focussed beam of accelerated matter for use as a space born weapon. No matter what, you have to use two equations to work out the momentum available for transfer. You have mass, and you have energy, and you have to conserve momentum as a vector. The idea that a directed matter nuke can deliver higher momentum ironically fails on exactly the same conservation law. Momentum must be conserved. The initial momentum of the nuke is zero, and no matter what you do with your directed mass jet, its momentum can only be as large as the momentum of matter ejected in the opposite direction by the nuke. Secondly, momentum is only m * v. Whereas energy is mv[sup]2[/sup]. The nuke delivers a fixed amount of energy. The absolute best possible momentum change of the asteroid is not with a small amount of very high speed mass, but a very large amount of low speed mass. You need to optimise energy transfer to the asteroid in order to get maximum mass to blast off it back at you. Not deliver the minimum momentum by hitting it with a jet of super fast but low mass.
I would guess there is a lot more to the mechanics of optimising the mass ejection from the asteroid than I am talking about, but it is going to be pretty clear that the mechanical properties of the rock and energy loss mechanisms within it are going to be a very substantial part of the issue.
We haven’t even gotten to the nuclear weapon part yet. I can speak to those.
Tripler
There is no such thing as a nuclear shaped charge.
I will say, though, Francis Vaughan, that yours are the most logical and reasoned posts I’ve seen in a long, long time. Thank you!!
Trip
No, I remain highly sceptical of a whole range of these designs. The Casabla Howitzer seems to be something that is claimed to exist as a precursor weapon before X-Ray lasers, but everything you find on the internet is fanciful discussion of a sci-fi weapon, with the most hand waving arguments imaginable about how it works.
The claim seems to be that a jacket of depleted uranium is capable of reflecting the x-rays and gamma rays from the very young fireball enough to make the tungsten plate accept a significant fraction of the entire nuke’s energy and produce what is indeed essentially a shaped charge. I call BS, but for the sake of the argument here I’m letting it slide.
Agreed.
I’ve had to pull up the citations, but there are basic design features that are not addressed. I’d speak to them, but I like my job and want to avoid prison.
Tripler
However, I hear Leavenworth is beautiful in the springtime.
If you do a Google Books search, you’ll find other references, including in congressional reports on the Joint Committee on Atomic Energy. It may not be something that could actually work in the real world, but it is something that was taken seriously by the US government at some point.
I did just that, and found those references. The trouble is that except for use of the name, there is little to join the claimed device and its operation to anything that has any provenance or suggestion of actually working. Given that X-Ray lasers never worked either, I think I can be forgiven for my scepticism that the design has any change of being of any value.
There are “reflectors” in nukes, but they don’t actually reflect, they sort of result in a short term increase in concentration during the scant nanoseconds before they vaporise. The idea that a weapon could be constructed that directs 80+% of its energy into a tiny thin beam is not on the cards (and the notion is clearly a compounded set of misreadings of the very thin material available.)
Yes. Thank you for taking the time to identify the details most of us (including me) were jumping over.
Once confusion & resistance to argument sets in the best cure is the one you took. Start with what’s agreed, identify all the unstated implicit spherical cows, make each of explicit, then adjust them into real world cows in the real world, not the Physics 101 world.
Thank you.
Granted.
But in the early atomic age many cockamamie things were taken seriously by the US government. See Supersonic Low Altitude Missile - Wikipedia & http://www.merkle.com/pluto/pluto.html for one “interesting” example.
But what does this have to do with the feasibility of redirecting asteroids which are threatening earth? Most of the papers I’ve seen describing nuclear methods involve using a stand-off detonation which either causes spalling or vaporization of a thin surface layer of material. The papers discussed to what extent the asteroid geology would affect this. They generally report it may be workable whether the body is a solid rock or a rubble pile, although knowledge of the composition would help calibrate the detonation point and number of devices required.
“Deflecting Asteroids by Means of Standoff Nuclear Explosions” (Gennery, 2014): home.earthlink.net/~dgennery/2004_1439.pdf
“Interception and Disruption” (Solem, 1995) : Interception and disruption (Conference) | OSTI.GOV
“On the Efficiency of Nuclear Explosives in Deflecting the Orbits of NEOs” (Yabushita, 1996): 1996EM&P...74..183Y Page 183
“Nuclear Explosion Near Surface of Asteroids and Comets -
General Description of the Phenomenon” (Shubin, et al, 1997): csc.ac.ru/news/1997_1/ae27.pdf
Many of these papers (inc’l various non-nuclear approaches) can be found in this single large PDF, “Proceedings of the Planetary Defense Workshop”, 1995: OSTI redirect | Research Library
Thank you. This seems to actually address what I was talking about.
There are 3 main mechanisms, then, not just 2.
- The nuke itself has some mass, it’s not just photons. If you put a tungsten plate on the side facing the asteroid, the mass of that tungsten is going to be converted to high speed gas that embeds in the asteroid and creates a small momentum change. But, like you say, that gas has very high velocity but little mass, so this effect is small. This is the principle by which the “orion drive” is supposed to work.
One mistake on this thread is mixing up the “casaba howitzer” nuclear shaped charge concept with the “project orion” nuclear pulse charge. They are very different proposals and one is science fiction and the other idea is solid.
The “project orion” charge is just an efficiently sized nuke, behind a some propellant, aimed at a plate on the back of a spacecraft. A diffuse *cone *of superheated gas is pushed by the explosion at the plate, and the spacecraft absorbs the shock smoothly and gains momentum. This is how I proposed redirecting asteroids, just without the shock absorber, since it doesn’t affect the momentum transfer in any way. This seemed to be where Stranger was going wrong.
The casaba howitzer is some super secret concept where somehow the nuclear explosion creates a thin and narrow beam of particles that can hit an enemy spacecraft thousands of kilometers away. In theory, this would have been a USSR orion drive spacecraft vs one built by the USA, dueling it out by ejecting these nuclear beam weapon charges into space, each combatant maneuvering quite dramatically with their orion drives. It would make for a cool science fiction story but I don’t know how you get that kind of focused, long range beam, maybe Tripler does. Both spacecraft would have been able to launch fully loaded from the ground, would each mass thousands of tons and be armored, have a crew of dozens to hundreds and lots of vacuum tube electronics…would have been pretty cool, shame about the nuclear fallout from the launches.
- The radiation from the nuke can boil material off the surface. If you have reasonably efficient nuclear charges and you set the nuke off from 0 distance away, some significant fraction of the energy goes into boiling. This is not affected by the deformation or shock waves, either. It’s just photons impinging on the surface, the material is now hot enough to exceed the vapor pressure in vacuum, it boils, and if the velocity is greater than escape velocity, it is lost forever.
Since you get a megaton for a few hundred kilograms of nuke, this is also pretty efficient. Basically a nuclear-(steam or metal vapor) rocket.
Both using the mechanism from Project Orion (high speed gas from a propellant package in the nuke) or using this boiling mechanism is much better than NASA’s current proposals, where it’s things like “park a solar-electric ion drive spacecraft real close and pull with gravity”. That’s going to be a glacially slow thing to do. Though, I suppose, over the long run if the ISP of the solar-electric spacecraft is high enough, it may work out to be the same actual effect on the asteroid.
- The third mechanism, I didn’t consider, is that apparently if you hammer a solid mass of iron hard enough, the shock waves create spalling. This can apparently let you create a really huge crater, sending solid fragments of the asteroid away into space. This is obviously a very efficient way to redirect an asteroid.
And, apparently, this maybe was what Stranger was talking about. This mechanism depends on the composition of the asteroid and certain asteroids won’t create a nice crater, if you hammer them, they’ll just soak the energy internally. This makes sense. Though even then, if you set a nuke off nearby, you’d get some momentum change because of effects (1) and (2)…but not enough to redirect a 10km+ asteroid in the small time you’d have before it ends most or all life on earth. And Stranger uses the word impulse to describe hammering the asteroid so it rings and creates this crater, which is actually defined as a momentum transfer. When it’s not necessarily.
Impulse has multiple definitions, one of which is p=mv…which is not necessarily even true. If you set off a bomb and a crater ejecting material is not formed, you do not get any momentum transfer at all. But the explosion itself is considered an “impulse” on the system.
Could you please run through the math on your suggestions? Start with some basic assumptions about speed, mass, and distance, and then work out the equations for exactly what would happen at the time of impact and for any changes in orbit.
Asteroid is 1.4×10^12 kilograms, a number I googled for a 1 kilometer asteroid. In order to push it’s impact probability down acceptably, a 1 cm/second change in velocity is needed. (I read that in a wired magazine article as being enough)
Nuclear pulse charge + small thrusters for guidance is assumed to be 300 kilograms, with a 150 kiloton yield. ISP is assumed to be 7500. (10k-20k from a paper on Project Orion, reducing it because you need a guidance system)
So how many nukes do you need?
(1.4×10^12)(0.01) = (7500*9.8)(300)(n)
n = 634. So 300 kilograms * 634 = 190,200 kilograms must reach the asteroid.
Falcon heavy stated payload to mars is 16,800. So you need 11 launches to do it.
Of course, you need to actually develop those nuclear pulse charges. Maybe you can’t get a really efficient one in the time you have and you need 10 times as many inefficient ones. Not all the rockets will make it. You may need a lot more dV than a “mere” mars injection in order to reach the asteroid in time. All these push the numbers into the less favorable territory, but it feels like it might work from this napkin analysis.
Bigger asteroids - 10 kilometer spheres - yeah, it’s not good. 1000 times the mass, this problem becomes 1000 times harder. Now you would need to hope you can eject craters in it, which in turn means it better be made of the right kind of material for that.
Exactly nothing. That was meant to be the point. It seemed that there was a background thread of talking about some sort of special nuke, that in the extreme was actually directional. Perhaps I should have ignored it. But it kept coming back.
You can modify the energy balance of a nuke between gamma rays and very fast neutrons, and as I noted earlier in this thread, this would seem to be a good idea as neutrons would be more penetrating and perhaps more apt to deposit their energy uselessly deeper in the rock. I suspect the use of tungsten wrappers or the like may have stemmed from this, and not from very dubious ideas about constructing a directional weapon. That was all.
Depending on what you conciser “smaller.” I found this interesting article suggesting that even 150m asteroids might be rubble piles.